Annoyingly that's general how performance related bonuses work. The goal posts are always moved and usually in one direction. Your boss probably didnt know where to set the Target at last year and may think it was achieved relatively easily so has therefore reassessed the position this year.
Edit: To actually answer your question I think you just need to start a discussion with your boss about it and just be honest. Ultimately a bonus is that so unless there is a specific bonus calculation included as part of your contract then you are bound by your bosses decision.